Effects of Low Phosphorus Stress on Shoot Architecture, Root System Architecture, and Phosphorus Use Efficiency in Pepper (Capsicum annuum L.)

This study aims to investigate the effects of low phosphorus (P) stress on shoot architecture, root system architecture (RSA), and phosphorus utilization efficiency (PUE) in pepper (Capsicum annuum L.), to reveal the underlying physiological and molecular adaptation mechanisms, and to elucidate the regulatory mechanism of low-P tolerance, thereby providing theoretical support for pepper germplasm improvement and phosphorus-reducing cultivation.

Three pepper cultivars, Zhangshugang (S8), Zunla-1, and 8214, were subjected to five phosphorus levels: phosphorus deficiency (P0, 0 µmol·L^-1), low phosphorus (P20, 20 µmol·L^-1), medium phosphorus (P60, 60 µmol·L^-1; P120, 120 µmol·L^-1), and normal phosphorus (P200, 200 µmol·L^-1). Shoot architecture, root system architecture, photosynthetic pigment content, photosynthetic characteristics, nitrogen, phosphorus, and potassium nutrient allocation, and acid phosphatase activity (Apase) were compared. Additionally, qRT-PCR was used to analyze the expression of genes related to shoot and root architecture development and phosphorus response.

Low phosphorus stress (P20) significantly affected shoot architecture development in pepper, resulting in thinner stems, shortened internodes, reduced lateral buds, and inhibited branch elongation. Concurrently, the inhibited shoot growth in pepper was accompanied by leaf abscission, leading to significant decreases in photosynthetic pigment content and photosynthetic performance. Regarding RSA, low phosphorus stress induced root architectural remodeling. Although primary root length was inhibited and lateral root formation decreased, resulting in significant reductions in root length, root tip number, root surface area, root volume, and average root diameter, the plants likely optimized resource allocation by preferentially allocating photosynthates and nutrients to the roots, leading to significant increases in root-to-shoot ratio and root-to-shoot phosphorus allocation ratio. Comparative analysis among cultivars revealed that under low phosphorus stress, Zunla-1 exhibited significantly higher low-P tolerance coefficients for total root length, number of root tips, chlorophyll content, net photosynthetic rate, and leaf phosphorus content compared to Zhangshugang (S8) and 8214, indicating stronger low-P tolerance. Its tolerance strategy involved optimizing RSA by increasing lateral root number and promoting root elongation, while maintaining higher levels of photosynthetic pigments, photosynthetic efficiency, and leaf phosphorus content. Physiological and molecular analyses indicated that the adaptive response of pepper involved coordinated physiological and molecular regulation. On the one hand, low phosphorus stress significantly upregulated the expression of phosphorus signaling and transporter genes (CaSPX1, CaSPX3, CaPHT1;7) and APase (CaPAP15, CaPAP17), thereby enhancing APase and PUE. On the other hand, low phosphorus stress also significantly upregulated the expression of the abscisic acid degradation gene (CaCYP707A1) and strigolactone biosynthesis genes (CaCCD7, CaCCD8), which are associated with shoot and root architecture development.

Low phosphorus stress may upregulate the expression of strigolactone biosynthesis genes (CaCCD7, CaCCD8) and the abscisic acid degradation gene (CaCYP707A1), thereby inhibiting shoot branching, optimizing RSA, increasing root-to-shoot ratio, and preferentially allocating photosynthates to roots. Meanwhile, it activates the phosphorus starvation response pathway to enhance APase and PUE, thereby systematically improving the adaptability of pepper to low phosphorus stress.